1. Field of the Invention
The invention relates generally to methods for processing signals received from satellites within a Global Navigation Satellite System (GNSS) for example, Global Positioning System (GPS) receivers, and more particularly to a method for computing precise relative locations using differential GNSS/GPS techniques for all observations measured by a rover GNSS receiver, and not just those observations that are common to base and rover GNSS receivers.
2. Description of the Related Art
Evolution of GPS-only systems to a multi-constellation Global Navigation Satellite System (GNSS) employing GPS, GLONASS, Galileo, Compass and others has become a reality. This has a number of implications when it comes to differential-based positioning (commonly called DGPS or DGNSS), particularly when it comes to mixing legacy equipment with newer equipment, or when equipment is not compatible with respect to the types of GNSS signals measured and processed.
Differential GPS/GNSS techniques have been successfully applied for a number of years. These techniques, for example, enable accurate real-time positioning of a rover receiver relative to a base receiver. This positioning includes code-only or carrier-smoothed-code differential techniques that result in sub-meter accuracy, such as those employed while operating with older RTCM 104 messages. They include carrier phase based techniques that facilitate centimeter-level real-time kinematic (RTK) positioning and employ single- or double- or even triple-differencing. Differential GPS is also the underlying technology behind Satellite-Based-Augmentation-Systems (SBAS) such as the FAA's Wide Area Augmentation System (WAAS) system as well as commercial Wide Area Differential (WADGPS) services such as those provided by OmniStar.
Differential GNSS (DGNSS), as its name implies, requires that data be differenced. One of the most useful differences in DGNSS, and therefore a widely used difference, is that of differencing two similar observations of a satellite ranging signal where one observation is made at a base GNSS receiver and another is made at a rover GNSS receiver. This type of difference, commonly referred to as the single-difference, removes common-mode errors (i.e. errors seen by both base and rover receivers) such as satellite orbit errors, satellite clock errors, and atmospheric errors that arise as the signal passes through the ionosphere and the troposphere. The remaining sources of error that result when employing single-difference techniques are those that are unique to the receiver, such as receiver noise and multipath. These remaining errors are often small in comparison to the common-mode errors, especially when carrier-phase observations are employed.
As is commonly understood, the use of a “differential corrector” is really just a form of single differencing where differencing happens implicitly as the differential correction is applied at the rover. In the case of WADGPS, data from a multitude of base receivers are processed together to form differential corrections, or constituents thereof, tailored to a particular rover location. The WADGPS SBAS systems that are prevalent today send data that are used to construct differential correctors, rather than the correctors themselves.
It is well known that there are advantages to processing signals from multiple constellations of a Global Navigation Satellite System (GNSS), including GLONASS, Galileo, and Compass signals, as opposed to processing GPS-only signals. The advantages are particularly related to increased numbers of observations which improve robustness, accuracy, and speed of ambiguity convergence. As such, the gradual migration from GPS-only to multi-GNSS capability has created a need for dealing with the wide variety of signals arriving from different satellite constellations. Different types of receivers will have different capabilities as far as their ability to track and process the multitude of GNSS signals. In a differential GNSS application, variations in receiver capability between base and rover may result in the inability to use all signals available for lack of appropriate matching pairs of observations. This situation is more likely when mixing different grades of receivers, receivers from different manufacturers, or older GPS-only receivers with newer GNSS receivers.
Take, for example, a system comprising a dual-frequency GPS base receiver that sends RTCM Version 3 code and carrier observations for the L1 and L2 GPS ranging signals to a rover GPS/GLONASS receiver. The rover measures code and carrier phase not only from GPS L1 and L2, but also GLONASS L1 and L2 signals. Prior to this invention, the rover, when performing differential positioning, could only use its GPS observations since GLONASS observations are of little use without matching observations from the base. This is actually a common problem since many RTK base stations are GPS-only, having been deployed before the recent shift towards GLONASS capability that has transpired as a result of the Soviet Union restoring GLONASS to full operational capability. And the net result is that the advantages that the added GLONASS satellites offer towards computing a robust, cycle-slip free location or solving RTK ambiguities cannot be realized.
Another example would be the use of a dual-frequency (L1/L2) GPS rover receiver with a single-frequency (L1) GPS base station. It is well known that dual-frequency RTK methods result in much quicker integer ambiguity resolution than single-frequency methods and are thus more desirable. A farmer, for example, using a GPS guided vehicle for precise planting of crops does not want to wait five to ten minutes after driving the vehicle under a tree to re-acquire RTK and continue planting. But this is often the case with single-frequency RTK. If the GPS base deployed in this scenario transmits only single-frequency L1 observations (or corrections), the dual-frequency rover on the tractor must, prior to this invention, rely only on single frequency RTK methods and essentially discard its L2 observations and its ability to do quick (or instant) ambiguity solutions.
Another situation is that of mixing GLONASS observations from different manufacturers' equipment, including receivers and antennas. It is well known that the Frequency Division Multiple Access (FDMA) methods of GLONASS are subject to frequency-dependent group delays and that these group-delays vary for different types of receivers and antennas due to variations in circuit design and design components. In RTK, these group-delays cannot be ignored, as they cause carrier phase errors at the centimeter-level or more. Group delays are not an issue when processing CDMA signals such as those from the GPS, Galileo or Compass systems since these signals are grouped into sets that share a common frequency and therefore exhibit a common group-delay. The common group-delay appears simply as receiver clock error for a given frequency of signal. Thus, to overcome the receiver and satellite-dependent group delay problem of GLONASS, one manufacturer's rover GNSS receiver may choose to ignore the GLONASS observations from a different manufacturer's base GNSS receiver, resulting in the same disadvantages as if the base did not provide GLONASS observations at all.
Yet another example occurs with a WADGPS system, such as the FAA's Wide Area Augmentation System (WAAS), when a receiver operates partially outside the coverage area. In this situation, differential correctors may be available for some satellites in view, but not all satellites. Using only satellites for which differential is available results in a higher Dilution of Precision (DOP). Consequently, the solution is more likely to suffer sudden loss of accuracy if one of the satellites critical to maintaining a lower DOP is suddenly blocked. Clearly, it is desirable to form a solution having the mathematical strength of all satellites in view when operating around objects that might block satellite signal reception as opposed to a solution that utilizes only the satellites for which differential is available.
To make the aforementioned problems worse, some GNSS receivers will mirror the use of velocity observations with the use of code or carrier phase ranging observations. That is, if the rover lacks base range observations to perform differencing on a particular channel, it ignores all observations on that channel, including velocity-related observations such as Doppler and delta-carrier phase. By ignoring velocity observations, the GNSS receiver is essentially penalizing itself twice for the lack of matching base-rover observation sets.
The methods of U.S. Pat. No. 6,397,147, which is assigned to a common assignee and is incorporated herein by reference, provide a unique and useful approach to compute differential corrections when there is no base station present. Briefly, the GNSS receiver first operates as a base station, computing differential correction terms, and then acts as a rover, using these correction terms. The problem with the approach, however, is that the user is forced to return occasionally to a place of calibration to re-calibrate the differential corrections; or else the user must be willing to live with a drift in the computed location estimate that arises when differential is not re-calibrated, or is calibrated using a non-precisely known location, such as its current location.
What is needed then is a system and method designed to take advantage of having a partial set of differential correction terms available rather than a complete lack of differential correction terms. Specifically, it is desired to have a method to take full advantage of the well-known differential GNSS approaches deployed today, but then gain the added benefit of the DGNSS methods disclosed in U.S. Pat. No. 6,397,147 when dealing with observations for which differences cannot be formed, as for example when experiencing a lack of commonality for some, but not all, base and rover observations.
An object of this invention is to provide such a method. This method allows all observations to be incorporated into a differential GNSS solution, not just those common to the base and the rover. A further object is to provide additional differentially corrected observations that can potentially reduce the time required to solve integer ambiguities, especially when recovering from a momentary loss of RTK. It is further object to provide extra observations so as to improve the performance of methods that use redundancy of observations to detect cycle slips in GNSS carrier phase observations. Yet another object of this invention is to provide a set of differential correctors that is locally generated in the rover receiver using the rover's own phase observations, with the correctors being referenced to the rover's, rather than the base', location. Such differential is effectively zero-baseline differential and is thus insensitive to atmospheric variations seen across long baselines when used for such purposes as re-acquiring ambiguity lock. It is a further object of the invention to provide differential that can bypass the effects of receiver-dependent, group-delay variations.